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nonenzymatic cleavage governed by glutathione, a tripeptide thiol antioxidant most concentrated in the liver.58 This cleavage leads to bioconversion to 6-mercaptopurine, a purine analogue that acts as a chemotherapeutic agent interfering with the synthesis of nucleotides, thereby inhibiting T-cell proliferation. The rate of the bioconversion shown in Fig. 3.13 is a major contributor to the release rate of this pro-drug-based dosage form.

New water-soluble prodrugs of an HIV protease inhibitor were tested recently; these prodrugs contain two linked units, a solubilizing moiety, and a self-cleavable spacer59 (Fig. 3.14). These prodrugs convert to the parent drug not enzymatically but chemically via intramolecular cycliza-tion through imide formation in physiological conditions. The release rate of the parent drug is controlled by the chemical structure of both the solubilizing and the spacer moieties.